1. Field of the Invention
This invention arises in the area of analytical equipment and instrumentation for the determination of moisture contents in solid materials. This invention is of particular interest in moisture level determinations of foods.
2. Description of the Prior Art
The moisture content of fruits, vegetables, and other foods or commodities is critical for various reasons, both regulatory and practical. The moisture level affects the shelf life, for example, and is a measure of the tendency of particular foods toward spoilage. The moisture level also affects the degree to which certain foods can be processed. In some cases, moisture levels are a factor in consumer preference as well.
Many moisture level determinations in current use operate by passing an electric current or other signal through a sample of the material and measuring the resistance of the material to the current or signal. The resistance generally increases as the moisture level drops. In other methods, infrared spectroscopy is used to measure absorption at a wavelength that is indicative of water content. Still other methods utilize atomic absorption, and further methods use chemical extraction processes.
Many of these methods are of limited accuracy, others are so costly as to be impractical, and some cannot be performed in the environments where testing is needed. One of the causes of inaccuracies is a lack of consistency of the physical properties of the material being tested. Foodstuffs with thick skins, fibrous flesh, or seeds, for example, are difficult to test in their native state due to differences between the water contents of these parts and those of the remainder of the material. When the foodstuffs are processed to prepare for moisture testing, the processing is often performed in an inconsistent manner, thereby introducing variations into the test results.
Prunes and raisins are examples of food materials in which moisture levels are critical. The industry has therefore adopted standardized methods prescribed by the Association of Official Analytical Chemists (AOAC). One such method is “AOAC Official Method 972.20, Moisture in Prunes and Raisins—Moisture Meter Method,” published in the AOAC Official Methods of Analysis (1995), “Fruits and Fruit Products,” Chapter 37, p. 4. This method requires use of a dried fruit analyzer of a specific type, and involves chopping or grinding a sample of the fruit, manually packing the ground fruit into a Bakelite cylinder that is part of the analyzer and that contains a bottom electrode, and doing so in such a way that the ground fruit is tightly packed in the cylinder and particularly around the electrode. A top electrode is then lowered onto the cylinder and pressed into the ground fruit. A thermometer is then inserted into the cylinder. The analyzer is then plugged into an AC outlet and the current adjusted. Readings are taken both from the analyzer and the thermometer. Despite the standardized nature of this method, significant human error is introduced into the readings since so much of the procedure is performed by hand. The manual filling of the cylinder, for example, as well as the manual compression of the sample around the electrodes, are difficult to duplicate to any degree of precision. Confirmation of the test results from different operators by more accurate but tedious vacuum oven determinations has shown that samples with the same moisture level can produce results by this method that vary by as much as 8%.
Another method prescribed by the AOAC is “AOAC Official Method 934.06, Moisture in Dried Fruits,” likewise published in the AOAC Official Methods of Analysis (1995), “Fruits and Fruit Products,” Chapter 37, p. 4. According to this method, a 5-10 g portion of a prepared sample is spread over the bottom of a metal dish, and the sample is then dried on the dish for 6 hours at 70° C. at reduced pressure under a flowing atmosphere of air that has been dried by bubbling through sulfuric acid. The dish is then covered, cooled and weighed. When raisins are tested by this method, the procedure requires the sample of raisins to be mixed with finely divided asbestos, then the mixture evaporated on a steam bath. It should be evident from this description that the entire procedure is replete with opportunities for operator error.